Flouropolymers are melt-processable resins that are formed into polymer structures by many different processes, such as extrusion, injection molding, fiber spinning, extrusion blow molding and blown film. They are also used as polymer processing aids due to their low surface energies and phase behaviors.
Flouropolymers, and especially polyvinylidene fluoride polymers and copolymers often have a white color. In the manufacture of fluoropolymer articles, thermoforming processes are often used, which often lead to undesirable discoloration of the fluoropolymer in the final product.
Several methods have been proposed to reduce discoloration of fluoropolymers during the processes for manufacturing articles. U.S. Pat. No. 3,781,265 describes the synthesis of poly (vinylidene fluoride) resin having good thermal stability by polymerizing VDF in suspension using diisopropyl peroxydicarbonate as the initiator and 1,1,2-trichlorotrifluoroethane as the polymerization accelerator. The synthesis of heat resistant PVDF by an emulsion process using ammonium persulfate as the initiator and methyl/ethyl acetate as the Chain transfer agent is reported in JP 58065711.
The use of special chain transfer agents has been reported to provide improved whiteness in PVDF synthesis, such as trichlorofluoromethane in U.S. Pat. No. 4,569,978 emulsion polymerization; Dialkyl ethers in JP 01129005 suspension polymerization; ethane in emulsion polymerization in U.S. Pat. No. 6,649,720; and HCFC-123 in emulsion polymerization in EP 655468.
U.S. Pat. No. 6,187,885 describes improved color using copolymerization of vinylidene fluoride (VDF) with hexafluoropropylene (HFP). According to this invention 1-20% HFP was added when 50-90% of VDF was already charged into the polymerization reactor.
EP 816397 describes improved resistance to heat-induced color distortion by a reduction of impurities, using a perfluoropolyether as the surfactant.
Suspension polymerization of VDF using organic peroxide initiators has been reported in JP 02029402. The application claims that pH treatment of the reaction mixture with NaOH yielded a milky white product that was resistant to discoloration at high temperatures.
Post-treatment of the fluoropolymer with sodium acetate for improved resistance to discoloration is described in US 2004225096.
Surprisingly it has been found that a fluoropolymer composition can be produced having excellent whiteness even after melt processing, by producing a multi-phase composition having a polyvinylidene fluoride continuous phase and a non-continuous phase having an average domain size of 20-900 nm, and a refractive index mismatch of between 0.007 and 0.07 between the phases.